Dementia 1

Prior to my current placement I have had very little to do with patients with dementia. It has been quite an eventful learning experience.
Obtaining consent can present a challenge with these patients.
After introducing myself and my role as a physio I asked the patient if she would be happy to come for a walk with me. To which she replied - Thank you so much but I can't as I'm going out for tea. Still trying to coax her into walking and with the sun pouring in through the window she responded with "don't be ridiculous, I'm not going for a walk at this time of the night". Then she asked me "what are you going to do?" to which I replied "I'm going to go for a walk with you". The patient stood up and said "thank you , how lovely of you" and proceeded to walk with me.
Can/should I assume implied consent because she chose to walk after I told her too?
I think it would be very easy (due to time pressure/frustration/ lack of patience/ having a bad day) just to walk away and record patient declined to walk but considerable care/attention needs to be taken to make sure that a client with dementia does not miss out on necessary treatment because of issues of consent.